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The Sediment Management Plan has categorized Combined Sewer Overflows (CSOs) into five categories. The locations of CSOs that fall into each category are shown in the map below.

Map of KC CSO sediment strategy categories Source: Sediment Management Plan Update Fact Sheet , Fall 2018

King County CSO Sediment Strategy Categories

map-legend-1-no-further-action

No cleanup is needed at these CSO sites. These fourteen CSOs are mostly located in the central basin of Puget Sound and Lake Washington.

map-legend-2-additional-monitoring

At these five CSOs, the data does not show the need for cleanup, but modeling suggests we keep an eye on them for possible problems.

map-legend-3-cleanup-needed

According to our data and models, there is one CSO that is contributing to sediment pollution that will require cleanup.

map-legend-4-area-wide-investigation

Five CSOs in Lake Washington Ship Canal/Lake Union are sited in highly developed areas where sediments have been impacted from multiple sources of contamination. Because of this, they need to be evaluated as part of an area-wide investigation to determine how to proceed. While contamination was measured in surface sediment, our modeling does not show that the CSOs created the problem. King County will participate as one contributor in part of a larger cleanup of the polluted waterway.

map-legend-5-part-of-existing-cleanup

At these locations, KC WTD did not necessarily create the problem, but they are located in more broadly polluted areas. Any contribution we have made and potential for recontamination will be addressed by a cleanup that is already planned or in process.

Fourteen CSOs and two CSO treatment plant outfalls are being addressed as part of an existing cleanup or through the original SMP. Twelve of these are part of the CERCLA cleanup process for the Lower Duwamish and East Waterway.